Poor balance is responsible for a significant proportion of falls and injuries in older persons, and interventions to reduce falls and injuries are limited by our current understanding of balance. The proposed research plan will investigate the importance of ankle strength and ankle torque in the maintenance of balance function in older persons, specifically focusing on understanding postural responses to backward falls. The long range goal of the applicant is to develop kinetic models of dynamic balance. The applicant will perform two experiments to test the importance of ankle torque in dynamic balance. The first experiment is a cross-sectional study of persons over 75. In the second experiment, the applicant will work closely with Gail Dalsky, Ph.D., on an intervention study testing the effects of brisk walking and stair climbing on femoral bone density. The applicant will test if the exercise program will improve dynamic balance, and will measure the change in response kinetics which may explain the improvement.